1. Field of the Invention
The present invention relates to power supply circuitry for an inductive heating element. A fixing apparatus of the induction heating type may be incorporated in an image forming apparatus, and the power supply circuitry may be used to supply power to an inductive heating element in such fixing apparatus.
2. Description of Related Art
The image forming apparatus generally contains a fixing device for fixing a toner image transferred to a recording material. As the fixing device, a heating type device using a ceramic heater or a halogen heater has conventionally been used in many cases. Recently, an electromagnetic induction heating type device has begun to be used (refer to Japanese Patent Application Laid-Open No. 2000-223253).
FIG. 12 illustrates a simple frequency control method employed for power control of a power supply unit, which supplies power to a fixing device of the induction heating type. In steps 4001 and 4002, detected power P is compared with target power Po. In the case of P>Po, then in step 4005, the frequency is increased by a predetermined value fa. In the case of P<Po, then in step 4004, the frequency is decreased by a predetermined value fb. In the case of P=Po, then in step 4003, the frequency is maintained.
FIG. 13 illustrates a simple frequency control method employed for temperature control of the fixing device. In steps 5001 and 5002, a detected temperature T is compared with a target temperature To. In the case of T>To, then in step 5005, the frequency is increased by a predetermined value fa. In the case of T<To, then in step 5004, the frequency is decreased by a predetermined value fb. In the case of T=To, then in step 5003, the frequency is maintained.
FIG. 14 illustrates a relationship between a driving frequency f and power P. As illustrated in FIG. 14, maximum power Pmax is supplied to a coil at a resonance frequency f1. Characteristically, supplied power is reduced when the frequency changes to a high-frequency side or a low-frequency side relative to the resonance frequency f1. Thus, it is possible to achieve power control by controlling the driving frequency f within a frequency range fh above the resonance frequency f1, in which range the power-frequency characteristic has a slope. It is also possible to control the power by controlling the driving frequency within a frequency range f1 below the resonance frequency f1.
More specifically, in a frequency control system, to reduce power, the driving frequency for a switching element, which is used to supply power to the coil, is set higher than the resonance frequency. However, when the driving frequency becomes higher than the resonance frequency, switching losses of the switching element may increase. Losses are particularly conspicuous when a large-power operation is performed in a state in which the driving frequency deviates from the resonance frequency.
Moreover, in a DC voltage control system for controlling power only based on a change in DC voltage supplied to the switching element, both a boosting circuit and a de-boosting circuit are required, thus leading to a great increase in production cost and circuit size.